Advanced multimedia services continue to drive requirements for increasing data rates and higher performance in wireless systems. Current technologies for high performance communication systems, such as those specified by the European terrestrial digital video broadcasting (DVB-T) standard and the Japanese integrated services digital broadcasting terrestrial standard (ISDB-T), employ communication methods based on Orthogonal Frequency Division Multiplexing (OFDM).
As known to those of skill in the art, multipath interference presents a significant impediment to effective wireless communication. Due to different length transmission routes, multiple versions of a transmitted data signal arrive at a receiver with different delays. These variable transmission times can result in inter-symbol interference (ISI) when the different data signals arrive at the receiver simultaneously.
In OFDM multiple sub-carrier systems, a higher rate data signal is divided among multiple narrowband sub-carriers that are orthogonal to one another in the frequency domain. Thus, the higher rate data signal is transmitted as a set of parallel lower rate data signals carried on separate sub-carriers.
A received OFDM symbol in an OFDM system generally consists of both data and pilot synchronization information transmitted on the multiple sub-carriers multiplexed together and spanning multiple sample periods. Modulation and demodulation in an OFDM system uses an inverse fast Fourier transform (IFFT) at the transmitter and a fast Fourier transform (FFT) at the receiver. At the transmitter, a cyclic prefix of a section of the IFFT output for each OFDM symbol is typically appended to the beginning of the OFDM symbol as a guard interval (GI). The length of the OFDM symbol before adding the guard interval is known as the useful symbol period duration. At the receiver, the cyclic prefix is removed prior to the FFT demodulation by the appropriate positioning of an FFT window, with size equal to the useful symbol period duration, along a received sample sequence. The FFT demodulation transforms the window of received time domain samples, in the received sample sequence, to a frequency domain (OFDM) symbol.
A principle advantage of this type of communication system is that the lower data rate occupies a longer symbol period than in a higher rate single carrier system. The addition of the guard interval to each lower frequency symbol contains the dispersion caused by multipath within the longer symbol period, reducing ISI. OFDM systems also offer a number of other advantages relevant to wireless applications, including high spectral efficiency and the ability to compensate for poor channel conditions, including signal fade.
A significant aspect of OFDM systems is the use of channel estimating techniques to correct for changes in the sub-carrier characteristics. In pilot-based systems, a known symbol, or “pilot,” is transmitted at given sub-carrier frequencies and at given times. Since the receiver knows the transmitted symbol, any errors to the transmitted pilot due to sub-carrier conditions can be estimated and an appropriate correction calculated. Channel conditions for all sub-carriers and times can likewise be interpolated from the pilot information, allowing equalization of the signal and subsequent coherent demodulation.
Further details regarding the design of OFDM systems can be found in co-pending, commonly-assigned U.S. patent application Ser. Nos. 12/272,629, filed Nov. 17, 2008, 12/277,247, filed Nov. 24, 2008, 12/277,258, filed Nov. 24, 2008, 12/365,726, filed Feb. 4, 2009, and 12/398,952, filed Mar. 5, 2009, all of which are hereby incorporated by reference in their entirety.
Despite these benefits, a number of challenges related to OFDM system design remain, particularly for mobile, wireless applications. For example, phase noise resulting in modulation constellation rotation degrades the performance of an OFDM receiver. As will be appreciated, frequency mismatches between the transmitter and receiver can cause a loss of orthogonality between sub-carriers, resulting in inter-channel interference (ICI). These effects are magnified in the time-varying environment characteristic of mobile applications. For example, frequency shifts due to relative motion between the transmitter and receiver as a result of the Doppler effect create challenges in the design of an OFDM system for mobile applications.
Common phase error (CPE) is a manifestation of phase noise associated with maintaining frequency synchronization. While separate corrections to minimize ICI generally must be made for each sub-carrier, CPE is independent of the specific sub-carriers. A significant portion of CPE can be attributed to errors in the frequency oscillators, a problem that is exacerbated by attempts to reduce the cost of hardware, particularly in receivers. Fortunately, CPE largely can be compensated for by using differential detection techniques.
Another significant source of phase noise, herein termed “phase glitch,” is a phase shift that occurs as a result of changes in radio frequency gain. Such changes are triggered in the receiver when the signal becomes stronger or weaker. These variations in signal strength can be triggered when a receiver is moving. In its path of motion, it may enter or leave areas in which the desired signal is shadowed. Alternatively, it may enter or leave areas in which multiple reflections of the signal add or subtract, causing signal fading that covers the entire bandwidth of the signal. To obtain good signal to noise ratio while avoiding distortion, it is normally necessary for the receiver to change the receive gain to compensate for such changes in signal amplitude. Depending on the design of the receiver, such changes in gain are often accompanied by changes in the delay or phase shift through the receiver. Thus, when the gain is changed to maintain proper signal sizing, an undesirable sudden shift in the phase of the received signal may result.
In some applications, the differential detection strategies used to combat CPE can successfully compensate for phase glitch. However, these conventional techniques for estimating phase glitch require continuous pilots, wherein a pilot is included with every OFDM symbol. Even if continuous pilots are available, differential detection is not suited for mobile environments because the phase change cannot be estimated effectively when the sub-carrier transfer function is time-varying. Thus, the phase change associated with Doppler shifts due to motion between the receiver and transmitter is too large to filter with these techniques.
Therefore, it would be desirable to provide OFDM systems that accurately estimate phase changes in a mobile environment. Likewise, it would be desirable to estimate phase changes without requiring continuous pilots. It would also be desirable to estimate phase changes despite a time-varying sub-carrier transfer function.